The intracellular protozoan parasite Toxoplasma gondii is a wide-spread pathogen that can infect many species including humans, leading to birth defects and neurological disease in immunocompromised individuals. Infection of immunocompetent hosts is generally asymptomatic due to a robust type I immune response that controls acute infection and allow establishment of a chronic phase. During the chronic phase the parasite persists in cysts within brain and muscle tissues. Studies using the mouse model of infection have provided considerable insight into how immune responses control infection. Nevertheless, large gaps remain in our understanding of how the parasite interacts with the immune system. How does the parasite manipulate antigen presenting cell function? What types of antigen presenting cells display T. gondii antigens to T cells during priming? How do CDS T cells and the cytokine IFNy, which are known to be crucial for immune protection, actually control the growth of the parasite? What is the fate for parasites following CTL lysis of an invaded target cell? What is the role of CDS T cells in preventing the re-emergence of the parasite during the chronic phase of infection? By directly visualizing CDS T cells in the process of protecting from infection, we hope to gain insight into these questions. In this proposal, I describe experiments using Two Photon Laser Scanning Microscopy to perform real-time imaging of fluorescently labeled CDS T cells, parasites, and antigen presenting cells within tissues. The imaging approach will be complemented by conventional flow cytometric and functional assays of immune responses. In Aim 1, we will investigate the cellular dynamics of priming of naive CDS by parasite infected cells. In Aim 2, we will investigate INF-y production and CTL killing by effector CDS T cells. In Aim 3, we will examine the role of CDS T cells during chronic infection by examining the behavior of T cells and parasites in the brains of chronically infected mice. These studies should provide insights into how our immune systems protect us from potentially lethal infection with Toxoplasma, an NIAID Category B Priority Pathogen.